1. Field of the Invention
The present invention relates to a noise measuring device that measures a noise signal superimposed on a signal. More specifically, the present invention relates to a power supply noise measuring device that is mounted in a large scale integrated circuit (LSI), and that measures a noise signal superimposed on a power supply signal in the LSI.
2. Description of the Related Art
A complementary metal oxide semiconductor (CMOS) circuit employed in a recent LSI has an advantage of reducing power consumption but has a disadvantage of always generating a power supply noise during switching. The power supply noise greatly influences a delay design in the LSI. When a power supply noise is increased in amount, a delay variation is increased accordingly, with the result that it is necessary to widen a margin for designing timing. The margin widened by as much as the delay variation relative to a timing window that is reduced following acceleration of signal transmission within the LSI greatly influences realization of the signal transmission. It is, therefore, important how accurately the power supply noise amount can be grasped.
In the conventional signal transmission at a rate equal to or lower than a rate in MHz, even if the power supply noise amount is observed on a printed circuit board outside of the LSI, a quantitative evaluation of the noise amount can be performed almost accurately without an influence of a waveform rounding.
However, if the LSI operates at a high-speed equal to or higher a rate in GHz, a rate of the generated power supply noise is accelerated accordingly. If so, it is difficult to measure such a power supply noise on the printed circuit board or the like outside of the LSI. In recent examples, as shown in FIG. 1 (2003 Symposium on VLSI Circuits Digest of Technical Papers, 4-89114-035-6/03, FIG. 5), in Symposium on VLSI Circuits, Intel Corporation proposed a method for extracting a minor variation in a measurement target power supply relative to a reference power supply, and for measuring a power supply noise amount based on a result of a magnitude comparison between the measurement target power supply and the reference power supply. With this method, when the high-speed noise is to be measured, a sampling frequency itself of an output is increased. As a result, it is disadvantageously difficult to transmit the output itself. In addition, a strict restriction is imposed on measurement at a plurality of locations in the LSI. Thus, this method is not suited for acceleration. Besides, probing of a peak noise that suddenly occurs is also difficult, because it depends on the sampling timing.
As another example, as shown in FIG. 2 (ISSCC 2002/Feb. 5, 2002/Salon 10-15/9:00 AM, FIG. 11.2.1), there is proposed a method using a sampling oscilloscope circuit as reported by Takamiya et al. in ISSC2002. With this method, data on a cyclically changing signal is acquired while shifting a data sampling timing by one clock each based on the principle of the sampling oscilloscope. Therefore, a data cycle is extended by T/ΔT times (where T is the clock cycle and ΔT is a resolution for acquiring the data), and the data can be fetched to the outside as a low-speed output. It is noted, however, the noise is assumed as a periodic noise in this method, so that this method is ineffective for a periodic noise. A chance of acquiring the peak noise that suddenly occurs, in particular, is extremely slight, so that it is quite difficult to acquire the peak noise.
The invention disclosed in Japanese Utility Model Registration No. 3096469 relates to a noise measuring device including a first comparator that compares a power supply voltage with a first reference voltage; a second comparator that compares the power supply voltage with a second reference voltage; a logic gate that outputs a signal indicating whether the power supply voltage exceeds a range from the first reference voltage to the second reference voltage using a comparison result of the first comparator and that of the second comparator; and a counter that counts outputs of this logic gate. According to the invention disclosed in Japanese Utility Model Registration No. 3096469, however, a direct-current (DC) level change of the power supply itself is also handled as a noise. Further, the invention disclosed in Japanese Utility Model Registration No. 3096469 is intended to measure a noise outside of the LSI. The device, therefore, fails to include a portion designed so that the portion is mounted in the LSI.